1. Field of the Invention
The present invention relates to a slide part used in a lubricated environment, and more particularly, to a slide part with an amorphous carbon coating formed on its sliding surface.
2. Description of the Related Art
Slide parts with amorphous carbon coatings formed on their sliding surfaces have been under intense research and development in order primarily to lower friction at sliding points particularly of automobile parts. Thin film materials referred to as DLC (diamond-like carbon), i-C (i-carbon), hard carbon, etc. are commonly used for amorphous carbon coatings.
Since amorphous carbon coatings are amorphous structures with no grain boundaries (no clear crystal structures), they have features of high hardness, high toughness, and low frictional properties at the same time. In this viewpoint, amorphous carbon coatings are regarded as superior to crystalline hard coatings, such as TiN (titanium nitride) and CrN (chromium nitride), in durability against mechanical wear. Therefore, slide parts with amorphous carbon coatings formed on their surfaces are expected to contribute to improving the fuel economy and extending the life of parts.
In general, amorphous carbon coatings are hard coatings composed of carbon atoms and hydrogen atoms. However, amorphous carbon coatings containing a ternary element have been proposed. For example, JP 2007-023356 A discloses a sliding layer of amorphous carbon containing more than 30 atomic percent (at. %) to less than 50 at. % of hydrogen and 1.5 at. % or more to 20 at. % or less of silicon. According to JP 2007-023356 A, the self-smoothing properties of the sliding layer and silanol formed on the sliding surface can lower the friction of slide parts.
WO 03/029685 A1 discloses an amorphous hard carbon coating containing at least one of silicon and nitrogen in an amount ranging from 1 to 50 at. %. According to WO 03/029685 A1, a high frictional slider which exhibits a stably high friction coefficient, favorable velocity dependent properties and a high wear resistance, and causes less damage to mating materials under wet sliding conditions where a drive-train lubricant is used can be obtained.
JP 2005-060416 A discloses a low frictional slider composed of members with sliding surfaces formed of DLC materials. The DLC materials of at least one of the members are composed of an amorphous carbon (a-C) material containing no hydrogen. The sliding surface between the members is lubricated with a lubricant composition containing a fatty acid ester ashless friction modifier and/or an alifatic amine ashless friction modifier. According to JP 2005-060416 A, the low frictional slider exhibits low friction properties and wear resistance superior to those of conventional slide parts composed of aluminum alloys and steel materials in combination with organomolybdenum compounds.
JP 2001-316686 A discloses a slider obtained by forming a hard carbon coating containing at least one metal element selected from among group IIb, III, IV, Va, VIIa, VIIa, and VIII elements in the periodic table on at least the surface layer of a substrate for use in a lubricant in an amount ranging from 5 to 70 at. %. According to JP 2001-316686 A, in a lubricated environment where there is a lubricant containing Mo-DTC (molybdenum dialkyldithiocarbamate) and Zn-DTP (zinc dithiophosphate), a lubricant additive coating is easily formed on the surface of the hard carbon coating, and the slider exhibits superior low frictional properties.
JP 2006-002221 A discloses a slide part with the outermost surface of its slide member coated with a chromium- and carbon-based diamond-like carbon coating containing 5 to 16 at. % of chromium. According to JP 2006-002221 A, the chromium-containing diamond-like carbon coating exhibits low frictional properties and favorable sliding properties even under unlubricated or less lubricated conditions.
JP 2008-195903 A discloses a sliding structure used in a lubricated environment where there is an organomolybdenum compound such as Mo-DTC even in a small amount. In the sliding structure, an amorphous carbon coating containing a hydrogen element is formed on the sliding surface of at least one of a pair of sliding members, and the amorphous carbon coating contains an element to inhibit the production of molybdenum trioxide by the organomolybdenum compound. More specifically, the amorphous carbon coating containing as the inhibiting element at least one element selected from among sulfur, magnesium, titanium, and calcium, for example, is formed so that the inhibiting element is provided into a lubricant via friction particles generated during sliding operations.
Kato et al. report a study on amorphous carbon nitride coatings (CNx-coatings) composed of carbon and nitrogen in Academic Document 1 entitled “Friction, Wear and N2-Lubrication of Carbon Nitride Coatings: a Review” by Koji Kato, Noritsugu Umehara, and Koshi Adachi, published in Wear vol. 254 (2003), pages 1062-1069. According to Kato et al., friction coefficients between silicon nitride (Si3N4) balls and CNx-coatings vary considerably depending on the kind of atmosphere gas, and they decrease significantly only in the nitrogen gas atmosphere.
Shinyoshi et al. and Jia et al. report studies on wear in amorphous carbon coatings in a lubricant containing Mo-DTC (molybdenum dialkyldithiocarbamate) in Academic Document 2 entitled “Wear Analysis of DLC Coating in Oil Containing Mo-DTC” by T. Shinyoshi, Y. Fuwa, and Yoshinori Ozaki, published in SAE 2007 Transaction Journal of Fuels and Lubricants vol. 116, paper number 2007-01-1969, and Academic Document 3 entitled “Tribological behaviors of diamond-like carbon coatings on plasma nitrided steel using three BN-containing lubricants” by Z. Jia, P. Wang, Y. Xia, H. Zhang, X. Pang, and B. Li, published in Applied Surface Science vol. 255 (2009), pages 6666-6674, respectively. According to these Academic Documents, Mo-DTC in a lubricant is thermally decomposed to produce molybdenum disulfide and molybdenum oxides, and molybdenum trioxide, in particular, is deeply involved in wear in amorphous carbon coatings. However, the views on the wear mechanism are different between the Academic Documents. Shinyoshi et al. maintain that amorphous carbon coatings wear by changing to carbon oxide gas due to redox reactions between molybdenum trioxide and amorphous carbon coatings. Meanwhile, Jia et al. claim that amorphous carbon coatings mechanically wear due to hard and sharp molybdenum trioxide.
In general, amorphous carbon coatings are known to have high hardness, low friction properties, and high wear resistance. Unfortunately, however, as reported in the above Academic Documents 2 and 3 and JP 2008-195903 A, amorphous carbon coatings can wear away significantly in a sliding environment where there is a lubricant containing molybdenum compounds including Mo-DTC (molybdenum dialkyldithiocarbamate) and Mo-DTP (molybdenum dithiophosphate), which are known as friction modifiers. At this stage, as can be inferred from the difference in views between Academic Documents 2 and 3, the wear mechanism has not yet been established in academic society.
On the other hand, it is an extremely important challenge to further improve friction properties and wear resistance of slide parts as typified by automobile parts in order to meet the growing needs for environmental protection and energy conservation. Although the invention described in JP 2008-195903 A attempts to solve problems assuming a wear mechanism similar to that described in Academic Document 2, its advantage has not proven fully sufficient in confirmatory experiments conducted by the inventors. This can be attributed to the fact that the wear mechanism has not been fully understood, among other things. In other words, it is crucial to shed light on the wear mechanism in order to solve the challenge.